Transvenous defibrillator leads are used for the correction of ventricular tachycardia and ventricular fibrillation. Leads of this type are intravenously positioned and are used to provide a variety of sensing, pacing and defibrillation functions. More than one electrode may be provided if it is desired to provide electrodes for defibrillation as well as for pacing and/or sensing. Typical leads are positioned into the right atrium and/or the right ventricle. Recently developed leads are positioned into the coronary sinus for use with atrial defibrillation systems.
Conventional transvenous defibrillator leads use a helically wound wire to conduct the electrical energy from the connector at the proximal end of the lead to the electrode near the distal end. The conductive electrode surface is most commonly provided by leaving a portion of the helically wound wire un-insulated and exposed, allowing it to contact or be in close proximity to the desired surface of the heart. Such exposed electrodes have a fundamental disadvantage with tissue ingrowth. The ingrowth and anchoring of tissue into the exposed coil makes the lead extremely difficult to remove, if removal is required (due to, for example, infection or dislodgment).
Various electrode coverings have been suggested to eliminate or minimize the tissue attachment to the electrode. U.S. Pat. No. 5,090,422 to Dahl et al. describes defibrillation electrodes provided with coverings of porous polymeric materials including polyurethane and polytetrafluoroethylene (hereinafter PTFE). The penetration of bodily fluids permits electrical conduction through the porous polymer. Dahl et al. teach that the electrode covering is greater than 0.25 microns thick and preferably greater than 2.0 mm thick, which results in a relatively large spacing between the electrode and the tissue to be stimulated and may require a longer time duration to re-wet following the transmission of an electrical discharge. In addition, a thick electrode cover may also increase the occurrence of gas build up following the transmission of an electrical discharge. Such a gas build up increases the electrical resistance through the cover. Thick covers also increase the stiffness and profile of the electrode, which are undesirable attributes, particularly when implanted into a coronary sinus. U.S. Pat. No. 5,755,762 to Bush teaches a similar porous PTFE electrode covering.
U.S. Pat. No. 5,609,622 to Soukup et al. teaches the construction of a porous PTFE electrode cover made conductive by loading the porous covering with a conductive powdered material such as graphite. Other insulating electrode leads and conductive electrodes incorporating porous polymeric materials are disclosed in U.S. Pat. No. 4,011,861 to Enger, U.S. Pat. No. 4,573,480 to Hirschberg, U.S. Pat. No. 5,148,806 to Fukui et al., U.S. Pat. No. 5,269,810 to Hull et al., U.S. Pat. No. 5,358,516 to Myers et al. and U.S. Pat. No. 5,466,252 to Soukup et al.
A relatively thin, porous, polymeric covering, suitable for use over a coiled implantable electrode, would have numerous advantages over the previously described art. For example, thin electrode coverings are typically more flexible, reducing abrasion and irritation to surrounding tissue. A relatively thin electrode covering will typically be more conductive and positioned closer to the desired tissue. A thin electrode covering can also provide a reduced profile or outer diameter, allowing placement within smaller vessels. An improved porous electrode cover would also incorporate a material, a wetting agent, tailored to allow wetting and electrical conduction by bodily fluids. Such an improved cover would also provide a barrier to tissue ingrowth and attachment, facilitating removal if required. Furthermore, removal is desirably accomplished without requisite for surgical dissection of the tissue from the covered portion of the lead.
A typical defibrillation electrode out-gasses and forms undesirable bubbles during rapid, repeated energy pulses. Bubble formation at an electrode is described by GH Bardy et al. in “Some factors affecting bubble formation with catheter-mediated defibrillation pulses,” Circulation 73, No. 3, 525–538, March 1986. The formation of bubbles at the electrode degrades the energy waveform. Excessive bubble formation can result in increased conduction resistance, which raises the energy required for defibrillation and increases local current density. It is desirable to provide a relatively thin electrode covering that has the additional capability of transferring repeated high-energy pulses without degrading the integrity of the covering. Thin coverings can readily diffuse bubbles through the porous covering materials during repeated defibrillation pulses.